Enrichment of iron from bauxite waste in chemical looping combustion

ABSTRACT

A method of recovering enriched iron fines from bauxite waste includes calcining particles of bauxite waste to form oxygen carrier particles, subjecting the oxygen carrier particles to chemical looping combustion at a temperature of about 950° C.-1,050° C. for energy production and to produce the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition and collecting the enriched iron fines.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application Ser. No. 63/303,084, filed on Jan. 26, 2022, which is hereby incorporated in its entirety by reference.

TECHNICAL FIELD

This document relates to a method and apparatus for the enrichment of iron from bauxite waste (red mud) using chemical looping combustion (CLC). According to the results from X-ray powder diffraction (XRD) X-ray fluorescence (XRF), and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), uniformly distributed iron in the red mud particles migrated outward to the surface during repeated redox cycles in a bubbling fluidized bed reactor, in which the size-reduced iron fines due to attrition are concentrated in filters. The concentrations of iron oxide in raw particles and attrition/iron fines are 43 wt. % and 83-87 wt. %, respectively. Since the attrition in CLC is inevitable, this recycled iron oxide can be a valuable by-product to compensate for the cost of CLC, whereas the bed materials with less attrition continue to participate in the CLC process for power (or energy) production. Moreover, the abundant bauxite waste can be processed in an eco-friendly manner for integrated power generation, carbon capture, and iron recovery by the proposed strategy in this work.

BACKGROUND

With the upsurge of greenhouse gas emissions, chemical looping combustion (CLC) shows significant benefits for clean power generation with in-situ carbon dioxide (CO₂) enrichment ready for sequestration for various kinds of fuel like coal, natural gas, and/or biomass. CLC is operated based on fluidized bed technology, where instead of direct air combustion, an oxygen carrier (OC) circulated between a fuel reactor (reducer) and an air reactor (oxidizer) carries the oxygen atoms forward for fuel combustion without the presence of nitrogen. Under such a scenario, at the fuel reactor exhaust, the produced CO₂ from flue gas after water vapor condensation is natively enriched and ready for utilization, e.g. enhanced oil recovery (EOR), or geological storage. Concurrently, the high temperature spent gas/flue gas produced can be used to produce steam for power generation.

One of the key setbacks for CLC operation is the attrition of OCs, resulting in a high makeup cost of OCs during CLC process. The size-reduced OCs that exit the reactors as attrition or iron fines are difficult to be returned to the reactor without further processing, e.g., re-calcination. Therefore, durable/attrition-resistant, reactive, and cost-effective OCs are imperative for commercializing CLC technology.

As an iron-based industrial waste from the bauxite process, red mud benefits low-cost OC fabrication for CLC (for power generation with in-situ carbon capture). It can be manufactured for as low as $113/ton, potentially reducing the OC makeup costs and minimizing or even eliminating the environmental impact from red mud disposal and storage. Previous research works on red mud oxygen carriers for CLC have focused on improving reactivity and oxygen transport capacity, but the valorization of attrition fines has not been reported. During CLC with iron-based OC, iron cycles between oxidation states under the reactor environments with implications for iron migration. While popular OC materials such as iron, nickel, and copper-based OC have been extensively studied, their migration of active elements, e.g., iron, nickel, and copper, and loss from attrition have received little attention. Several researchers have studied the iron migration mechanism in binary Fe₂O₃ and TiO₂/Al₂O₃ systems in fixed bed reactor.

Li et al. studied the effect of multiple redox reaction cycles on the migration mechanisms in the iron-based oxygen carrier and found that solid-state ionic migration plays a more significant role than the intraparticle gaseous migration. The inert, TiO₂, enhanced the oxygen diffusivity and reactivity by creating an oxygen vacancy in the Fe₂O₃—TiO₂ oxygen carrier. In the pure iron oxides, no oxygen vacancy was created in the bulk of the oxygen carrier, so the oxygen was mainly available in the superficial layer. Later on, Sun et al. reported the synthesis of core-shell structures by outward migration of iron in a mixture of Fe₂O₃—Al₂O₃ particles at 900° C. in a thermogravimetry analyzer (TGA) In contrast to the work of Sun et al., iron did not migrate outward in the mixture of Fe₂O₃—TiO₂ because oxygen anions diffuse inward and Fe cations remain in situ. These studies explained the iron diffusion in binary systems of Fe₂O₃—Al₂O₃; furthermore, researchers also reported the element diffusion in multi-component systems, e.g., natural iron ore.

Condori et al. recently reported the Fe migration to the outer layer of iron ore particles during the gasification of wood residue for syngas production in a 1.5 kWth continuous CLC reactor. Chen et al. recently reported the outward iron migration in ilmenite oxygen carrier at 950° C., composed of 51 wt. % Fe₂O₃ and 44 wt. % TiO₂, and other inert components (SiO₂, Al₂O₃, MnO). Since there was no iron migration reported in synthetic Fe₂O₃ (30 wt. %)-TiO₂ (70 wt. %) at 900° C. according to Sun et al., it seems that the other inert components or temperature may affect iron migration in the Fe—Ti system in which the particulate porosity and/or oxygen vacancy could play a significant role. Similarly, the abundant industrial waste, bauxite waste, or red mud, is composed of Fe₂O₃, AlO₃, and other compositions. Fe₂O₃ is the only active compound in red mud for redox reactions of the CLC process. The other impurities may affect the migration of iron in the particles. However, if the iron still migrates outward from the bulk to the surface of the red mud oxygen carrier, the enrichment of iron can be achieved. Furthermore, it is possible to extract the iron from bauxite waste during abrasive attrition in the CLC process by leveraging the iron outward migration phenomenon and an abrasion-dominating attrition mechanism.

Advantageously, it is possible to recover the valuable metal, iron oxides, with high purity from bauxite waste as a beneficial byproduct resulting from particle attrition, which is inevitable for CLC technology. In this scenario, industrial waste as an oxygen carrier is cost-effective because the active compound, iron oxide, can be enriched for iron or steel manufacturing.

SUMMARY

In accordance with the purposes and benefits described herein, a new and improved method is provided for recovering enriched iron fines from bauxite waste. That method comprises the steps of: (a) calcining particles of bauxite waste to form oxygen carrier particles, (b) subjecting the oxygen carrier particles to CLC at a temperature about 950° C.-1,050° C. to produce energy and the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition, and (c) collecting the enriched iron fines that are emitted from fluidized processes by filtering.

In one or more of the many possible embodiments of the method, the calcining of the particles of bauxite waste includes heating the particles of bauxite waste to a predetermined temperature for a predetermined period of time needed to form the oxygen carrier particles.

In one or more of the many possible embodiments of the method, the oxygen carrier particles have a size of 150 to 500 μm.

In one or more of the many possible embodiments of the method, the subjecting of the oxygen carrier particles to chemical looping combustion includes cyclically reducing and oxidizing the oxygen carrier particles to produce the enriched iron fines while generating energy. The reducing may include subjecting the oxygen carrier particles in a reactor bed to the presence of a reducing agent. The oxidizing may include subjecting the oxygen carrier particles in the reactor bed to heating in the presence of an oxidizing agent. The reducing agent may comprise carbon monoxide (CO), hydrogen (H₂), methane (CH₄), or carbon. The oxidizing agent may comprise air.

In one or more of the many possible embodiments of the method, the collecting of the iron enriched fines is done by filtering using, for example, a gas-solid separation device, such as a baghouse, an electrostatic precipitator, a cyclone or a combination thereof.

In one or more of the many possible embodiments of the method, the calcining of the particles of the bauxite waste is performed at a temperature of between about 1200° C. and 1300° C. Further, the calcining may be performed for about 5 hours.

In one or more of the many possible embodiments of the method, the heating of the oxygen carrier particles during the reducing and the oxidizing cycles is to a temperature of between about 950° C. and about 1050° C.

In accordance with an additional aspect, a method of recovering enriched iron fines from bauxite waste, comprises: (a) calcining particles of bauxite waste to form oxygen carrier particles, (b) subjecting the oxygen carrier particles to chemical looping combustion at a temperature about 950° C.-1,050° C. to produce a spent flue gas and the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition, (c) collecting the enriched iron fines from the spent flue gas, and (d) using the spent flue gas to generate steam for power generation.

In accordance with yet another aspect, a method of recovering enriched iron fines from bauxite waste, comprises: (a) subjecting the oxygen carrier particles made from bauxite waste to chemical looping combustion at a temperature about 950° C.-1,050° C. to produce a spent flue gas and the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition, (b) collecting the enriched iron fines, and (c) using the spent flue gas, following the collecting of the enriched iron fines, to generate steam for power generation.

In the following description, there are shown and described several embodiments of the method of power generation and recovering enriched iron fines from bauxite waste. As it should be realized, the method is capable of other, different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the method as set forth and described in the following claims. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic diagram of a chemical looping reactor.

FIG. 2A is a scanning electron microscopy (SEM) image of a pristine oxygen carrier (OC) particle.

FIG. 2B is a SEM image of a reacted oxygen carrier particle.

FIG. 2C is a SEM image of a redox or enriched iron fine.

FIG. 2D is a SEM image of a non-redox fine.

FIG. 3A is a graph illustrating the calculated terminal velocity versus particle size in a fluidized bed reactor.

FIG. 3B is a graph illustrating the particle size distribution of attrition fines in a fluidized bed reactor.

FIG. 4 is a graph illustrating reaction and non-reaction attrition rates of RM oxygen carrier in a fluidized bed reactor. The reaction attrition rate is always higher than the non-reaction attrition rate, suggesting that redox reactions dominate the attrition.

FIG. 5A is a graph showing the differential pressure and sample temperature of 148 redox cycles of RM OC in the fluidized bed reactor, which showed the stable temperature swing; but the differential pressure increased due to agglomeration at the superficial velocity of two times the minimum fluidization velocity.

FIG. 5B is a graph showing the differential pressure across the porous sample distributor and oxygen carrier particles. The sampling frequency is 2 points per second. (The 43rd-53rd hours/17th-28th cycles were listed here as an example). The calculated DP for 200 g of particles was 1728 Pa. The higher DP in the reactor was introduced by the sample distributor and agglomeration. Right: the temperature swing during reduction and oxidation due to the exothermal oxidation (FeO+O₂→Fe₂O₃, ΔH=−559 kJ/mol_O₂) and exothermal reduction (Fe₂O₃+CO→FeO, ΔH=−4.7 kJ/mol_CO₂), which caused the thermal stress inside the OC particles.

FIG. 6 is a graph showing the particle size distributions (PSDs) of RM-pristine and RM-reacted OCs. The PSDs for each sample were the averaged results of 31 testing times. The similar particle size distributions of pristine and reacted OC after 70 redox cycles at the superficial velocity of 3 times minimum fluidization velocity indicate the same hydraulic characterization of particle inside fluidized bed can be achieved. Hence, the RM OC “maintained” recyclability within 70 redox cycles. The minor peak of reacted RM OC is generated by abrasion attrition inside the reactor. The span of RM-pristine and RM-reacted are 0.90 and 1.12, respectively, suggesting that the particle size of RM-pristine OC is more uniform than RM-reacted. The RM-pristine OC has sieved size of 300-500 μm.

FIG. 7 illustrates the main chemical components of samples (Cr₂O₃ is introduced by erosion of reactor). Iron was enriched in the attrition fines.

Reference will now be made in detail to the present preferred embodiments of the method, examples of which are illustrated in the accompanying drawing figures.

DETAILED DESCRIPTION

A new method is provided for recovering enriched iron fines from bauxite waste while simultaneously generating energy for the power grid. The method may be described as including the steps of calcining particles of bauxite waste to form oxygen carrier particles and then subjecting the oxygen carrier particles to chemical looping combustion at a temperature above 1000° C. to produce (a) energy for the power gird and (b) the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition. This is then followed by the step of collecting the enriched iron fines for further processing.

Alternatively, the method may be described as including the steps of (a) calcining particles of bauxite waste to form oxygen carrier particles, (b) subjecting the oxygen carrier particles to chemical looping combustion at a temperature above 1000° C. to produce a spent flue gas and the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition, (c) collecting the enriched iron fines from the spent flue gas, and (d) using the spent flue gas to generate steam for power generation.

Alternatively, the method may also be described as including the steps of (a) subjecting the oxygen carrier particles made from bauxite waste to chemical looping combustion at a temperature above 1000° C. to produce a spent flue gas and the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition, (b) collecting the enriched iron fines, and (c) using the spent flue gas, following the collecting of the enriched iron fines, to generate steam for power generation.

The iron oxide in the oxygen carrier particles is about 43 wt % while the iron oxide in the enriched iron attrition fines is about 83-87 wt %. Advantageously, the concentration of iron oxide approximately doubles during looping combustion and the energy production process.

The calcining of the particles of bauxite waste includes heating the particles of bauxite waste to a predetermined temperature for a predetermined period of time needed to form the oxygen carrier particles. For example, and not to be considered limiting in any respect, the particles of bauxite waste may be heated to a temperature of between 1,200-1,300° C. for a period of about 5 hours. The resulting oxygen carrier particles may have a size of between about 300-500 μm.

The chemical looping combustion of the oxygen carrier particles may include cyclically reducing and oxidizing the oxygen carrier particles to produce the enriched iron fines while generating energy. This may be accomplished in a reactor bed of a type known in the art. Thus, the reducing may include subjecting the oxygen carrier particles in a reactor bed, such as a fuel reactor, to the presence of a reducing agent. That reducing agent may comprise carbon monoxide (CO), H₂, carbon, coal, natural gas or other fuels including solid and gas fuels. The oxygen carrier particles may be heated, cooled or maintained at a constant temperature during the reducing process.

The oxidizing of the oxygen carrier particles may include subjecting the oxygen carrier particles in a reactor bed, such as an air reactor to heating in the presence of an oxidizing agent, such as air. In some embodiments, where a single reactor bed is used for reducing and oxidizing the oxygen carrier particles, the reactor bed may be purged with an inert gas, such as nitrogen, between the reducing and oxidizing steps. In at least one embodiment, the cyclical reducing and oxidizing of the oxygen carrier particles uses a thermal heating cycle of about 950° C. to about 1050° C.

The collecting of the iron enriched fines may be done by filtering. This filtering may be done by using a gas-solid separation device such as a baghouse, an electrostatic precipitator, a cyclone or a combination of two or more of these devices.

Experimental Section Chemicals and Oxygen Carrier Preparation

Because red mud is composed of various components, to ensure the homogeneity of samples, the heterogeneous raw red mud (from an alumina company) was well-mixed by milling to a particle size of D90=18 μm in a roller with alumina beads, and then dried in an oven for 24 hours at 105° C., followed by calcination in a muffle furnace at 1250° C. for 5 hours, then cooled to room temperature to form clinkers. The clinkers were crushed and sieved, and the particle size of 300-500 μm was selected as the oxygen carrier (Pristine OC, hereinafter) for fluidization as Geldart B particles. The chemical compositions of raw red mud and pristine OC are listed in Table 1.

TABLE 1 The chemical compositions of raw red mud, pristine OC, and redox fine samples by XRF. Samples Fe₂O₃ Al₂O₃ SiO₂ TiO₂ Na₂O CaO SO₃ P₂O₅ MgO KO Cr₂O₃ wt. % Raw RM 43.43 23.84 12.01 7.87 6.65 5.14 0.62 0.29 0.08 0.07 0 Pristine 42.77 24.31 12.33 7.44 6.85 5.16 0.66 0.31 0.09 0.08 0 OC Redox 83.49 5.16 1.16 2.18 1.04 0.60 0.19 0.14 0.07 0.04 5.92 fine Non- 87.14 4.77 0.89 2.26 1.03 0.51 0.11 0.13 0.06 0.03 3.05 Redox Fine

The raw red mud was in a fully oxidized state prior to processing, and after calcination in air, the pristine OC remained fully oxidized. So, the two samples have the same components and concentrations. The active compound for CLC, Fe₂O₃, has a concentration of 43 wt. % in pristine OC.

Bench-Scale Fluidized Bed Setup

The fluidized bed setup was composed of gas supplies, preheater, furnace, reactor and filters. The reactor was made of highly alloyed austenitic stainless steel (310S) and had an inner diameter of 38 mm and a height of 70 cm, with a porous frit inserted as a sample holder at the ⅓^(rd) position from the bottom. The preheater minimized the temperature difference between switching gases, and a LabVIEW system controlled the gases. The reactor was equipped with a differential pressure gauge to monitor the particle fluidization behavior during the thermal oxidation and reduction. A schematic of the setup is shown in FIG. 1 , in which the attrition fines collected from the filters are much smaller than the oxygen carrier particles.

Redox Cyclic Tests

The total gas flow rate was 5 L/min during the oxidation, purge, and reduction cycles, and the superficial velocities were about twice the minimum fluidization velocities of particles in the reactor bed at 950° C. Specifically, the superficial velocities at experimental temperature during oxidation, purge, and reduction were 0.305 m/s, 0.299 m/s, and 0.299 m/s, respectively, while the minimum fluidization velocities during oxidation, purge, and reduction were 0.147 m/s, 0.145 m/s, and 0.145 m/s, respectively. 2.5 L/min Air balanced with 2.5 L/min N₂ for oxidation, and 1 L/min CO balanced with 4 L/min N₂ for reduction. 5 L/min N₂ was purged for 5 minutes between oxidation (20 minutes) and reduction (20 minutes). The total running time was 300 hours with 148 redox cycles. Specifically, the redox cycles were tested during the daytime, and the fines collected in the redox filter were named as redox fine (C), while the fines collected during air fluidization in the night were named as non-redox fine (D). 200 g of pristine OC (A) (See FIG. 2A) were loaded into the reactor, and redox fines (C) (See FIG. 2C) were collected from filter every 12 cycles/10 hours; while non-redox fines (D) (See FIG. 2D) were collected every 14 hours. After finishing the test, the reacted OC (B) (See FIG. 2B) and other samples were collected and characterized. Hereinafter, these samples are denoted as A, B, C, and D.

Characterization SEM-EDS

The samples were analyzed with a SEM (Hitachi S-4800, Japan) coupled with EDS (SciXR Micro Analysis, USA). The reacted OC particles and pristine OC particles with a size of 300-500 μm were mounted and polished to get the cross-section morphologies under SEM/EDS. The steps for cross-section preparation for SEM/EDS were:

-   -   a) The epoxy mixed with hardener (10:1 g/g) was poured into the         molds whose bottoms were covered with pristine OC/reacted OC,         waiting for 24 hours at room temperature for hardening;     -   b) The hardened specimens were ground and polished with a         grinder and polisher system (EciMet® 250, Buehler, USA), where         the grinding paper was 600 grit/P1200 (Grit size 15.3±1.0 μm),         and the polishing cloth was MicroCloth® by Buehler. The         polishing solution was Masterprep® suspension;     -   c) The polished specimens were sputtered with gold under the         sputtering system (Hummer®, USA) to increase electrical         conductivity for SEM/EDS scanning; and     -   d) The sputtered specimens were ready for SEM/EDS scanning.

XRD/XRF

The powder X-ray diffraction patterns of pristine RM OC (red mud oxygen carrier), reacted RM OC, redox fines, and non-redox fines were collected on a Rigaku SmartLab system (Cu Kα, 20-70°, 0.01°/step). X-ray fluorescence (XRF, Rigaku Primus IV, Japan) was used to analyze the chemical composition of the samples.

Calculation of Attrition Rate of OC

The attrition rate was calculated according to the following equation:

Attrition rate,%=(M _(f)/(M _(o) ×t ₁))×100%

where M_(f) is the mass of redox fines from the 1^(st) filter and t₁ is the duration of cyclic operation; or while M_(f) is the mass of non-redox fines from the 2^(nd) filter and t₁ is the duration of overnight fluidization. M_(o) is the total mass of bed material, and in order to maintain the total mass of bed material (M_(o)=200 g), additional pristine OC was fed into the reactor after each sampling according to the mass of collected fines.

The Mechanical Strength

To compare the recyclability of reacted particles to pristine OC, the mechanical strength of pristine RM OC and reacted RM OC after 148 cycles with the same particle size (300-500 μm) were tested by a Shimpo FGE-10× gauge, which measures the peak values in Newton when particles are crushed under a force load. Each sample was randomly tested 100 times to reduce the statistical error.

Results and Discussion Bench-Scale Bed Fluidization

During CLC operation, fluidization facilitates OC circulation. However, for a distributed particle size, the superficial velocity is typically larger than the terminal velocity for attrition fines, e.g., samples C and D, while the other particles, e.g., samples A and B, are fluidized in the reactor bed. The terminal velocities as a function of particle size are given in FIG. 3 a according to the equations of Kunii, Haider, and Levenspiel. FIG. 3 a indicates that if the particle size is less than 100 the particle will leave the reactor at a superficial velocity of 0.3 m/s, which is confirmed as depicted in FIG. 3 b , where the major particle size associated with the fines collected in the filters is less than 100 μm (D90=70 μm). The SEM micrographs (FIGS. 2A-2D) can further support such a conclusion, in which samples C and D collected from the filters are much smaller than samples A and B (300-500 μm). These fines were generated by the abrasion-dominant attrition of red mud OC particles in the fluidized bed reactor.

Attrition Rates

Generally, two models are used to describe particle attrition, including fragmentation and abrasion. Fragmentation generates secondary particles with a similar particle size to the parent particles, which is due to the collision/impact between particles and particles/wall. Abrasion generates fines from the surface or superficial layer of parent particles by surface wear, leading to the elutriation of these fines and a decrease in bed materials inventory. Mendiara et al. found that most synthetic OCs and natural iron ores experienced attrition by abrasion in their CLC unit at 800-1000° C. Moreover, Liu et al. found that the bulk fracture/fragmentation of the synthetic Fe₂O₃—Al₂O₃OC was dominated at 950° C. Thus, different OC materials and conditions can lead to contrasting explanations of the attrition mechanisms. However, the implications of OC attrition are unavoidable.

In this work, there were two alternating operating stages: the first was a repetitive reduction-oxidation cycle during the daytime, and the second was an air fluidization test overnight for the sake of safety precaution. Hence, two attrition rates are calculated. As shown in FIG. 4 , the first is reaction attrition rate, which is the attrition resulting from redox cycling that could be caused by thermal, chemical, and mechanical stresses. The second is non-reaction attrition due to fluidizing with air that is mainly from mechanical stress. The result shows that the reaction attrition rate is much higher than the non-reaction attrition rate, suggesting that the attrition is dominated by redox cycling reaction via chemical stress and thermal stress on the active metal bonds, lattice structure, and morphology during the chemical looping combustion. Specifically, the chemical stress is from the oxidation-reduction induced volume change in the particles. Furthermore, the thermal stress is from the temperature swing (ΔT=250° C.) during reduction and oxidation as indicated in FIGS. 5A-5B. The temperature change would create volume change and lead to attrition of particles. Therefore, the reaction attrition rate was higher than the mechanical attrition rate in the CLC process.

The overall performance of 148 redox cycles exhibited near-stable temperature change during reduction and oxidation, indicating that the overall reactivity of RM OC was stable. Furthermore, since the attrition became stable at a low rate<0.03%/hour after 100 cycles, the durability of OC can be interpreted at approximately 3300 hours that is in the reasonable range of OC recyclability; moreover, the crushing strengths of the particles with the similar particle size (300-500 μm) decreased from 12.61 N of for pristine OC to 6.97 N of for cycled OC, but the 6.97 N of crush strength of 148-cycles OC was still higher than most of the reported values. Furthermore, the similar particle size distributions of pristine and reacted RM OC after 70 redox cycles at superficial velocity of 3 times the minimum fluidization velocity, FIG. 6 , indicates the same hydraulic characterization of particles inside the fluidized bed can be achieved.

The Morphological and Elemental Variations of Oxygen Carriers

As mentioned above, attrition was dominant during the redox cycles. Under such conditions, surface morphologies and chemical changes are of interest. The edge of sample B (FIG. 2B) was found to be much less sharp than sample A (FIG. 2A), which is very likely due to the abrasion by rounding effects during the process. Moreover, the surface morphology of Sample A (pristine OC) was found to be more uniform than that of Sample B (reacted OC). The surface elemental analysis carried out using SEM-EDS, showed a uniform distribution of Fe, Al, Na, and Si in Sample A. In comparison, the distribution of the elements differs for Sample B. Specifically, there are two surface morphologies on Sample B: Fe-diluted porous morphology (which was identified as NaAlSiO₄ in the following XRD analysis) and Fe-enriched dense morphology (which was identified as Fe₂O₃). It is likely that the Fe-deficient area away from the surface resulted from the Fe-enriched outside scales peeling off the particles due to attrition and transported as fines, leaving behind a porous particle surface. To verify this hypothesis, cross-section characterization of SEM-EDS was further performed, and the relevant results are disclosed in the following sections.

The Cross-Section Morphologies and Elemental Distributions

The cross-section SEM-EDS was performed to confirm the aforementioned conclusions. Compared to Sample A, Sample B had a noticeable outer layer morphology, and the exterior layer was denser while the interior was more porous. Moreover, the exterior layer had a higher Fe concentration and a lower Al concentration than the interior part (Fe and Al are presented as they are the two dominant elements in red mud). In addition, SEM-EDS further showed that the Fe-enriched layer has a uniform Fe concentration, and the thickness of the Fe layer is about 5 As contrast, the pristine OC particle had a uniform distribution of Fe and Al from the surface to the bulk of the particle. On the other hand, the enriched-Fe layer is partly peeling off from the parent particle, leaving the porous surface exposed, which is the source of attrition. Therefore, the reduction-oxidation cycles in the CLC can form a core-shell structure for red mud oxygen carrier with Fe-enriched shell and an Fe-deficient core. Since the Fe-enriched shell experienced attrition, the fines should be enriched with iron oxides that can be collected in the filters during our process.

Chemical Components of Samples

To support the observation from SEM analysis, the attrition fines were further analyzed using XRF. The main chemical components of fines characterized by XRF are shown in FIG. 7 , and the other trace components are given in Table 1. As shown in FIG. 7 , compared to the raw red mud and pristine OC, the non-redox fine and redox fine possesses increased Fe but decreased Al, Si, e.g., 83 wt % Fe₂O₃ in the redox fine versus 43 wt % Fe₂O₃ in the pristine OC.

It is noted that Cr₂O₃ in the redox fine sample is not from red mud but introduced by the erosion of the fluidized bed reactor used for the experiment, which is made of SS310 (24-26 wt. % Cr and ˜50 wt. % Fe). Hence, the estimated Fe₂O₃ derived from SS310 is about 11.57% according to the 5.92 wt. % Cr₂O₃ in the redox fine. The revised iron oxide concentration from redox fine after excluding Cr₂O₃ (5.92 wt. %) and Fe₂O₃ (11.57 wt. %) is (83.49 wt. %−11.57 wt. %)/(100%−5.92%−11.57%)=87.17 wt. %. Similarly, the revised iron oxide from non-redox fine is (87.14 wt. %−7.45 wt. %)/(100−3.05%−7.45%)=89.04 wt. %. Hence, the Fe₂O₃ concentration increases by 103-107% compared to pristine OC. Moreover, the total iron content (Fe) of attrition fine samples is 58-61 wt. %, which is similar to high-grade ore with a cutoff grade of >60 wt. %, according to the Department for Energy and Mining of South Australia.

Proposed Mechanism of Fe Migration and Enrichment of Red Mud Oxygen Carrier

As previously discussed, Fe can migrate from the bulk to the outer layer of the RM. The related works from other researchers are summarized in Table 2.

TABLE 2 Literatures reporting the migration of Fe in oxygen carrier OC Active phase Reduction Oxidation Reaction unit Ref. Fe₂O₃—Al₂O₃ 30 wt. % Fe₂O₃ 25 vol. % H₂ + 25 vol. % TGA, 50 cycles, (Sun et 75 vol. % N₂ Air + 75 900° C. al., 2013) vol. % N₂ Fe₂O₃—Al₂O₃ 50 wt. % Fe₂O₃ 5 vol. % CO + 100 vol. % Fixed bed, 20 (Ma et 95 vol. % N₂ Air cycles, 900° C. al., 2019) Fe₂O₃—Al₂O₃ 50 wt. % Fe₂O₃ 20 vol. % CO + 100 vol. % Fluidized bed, (Liu et 80 vol. % N₂ Air 60 cycles, al., 2022) 950° C. Ilmenite 51 wt. % Fe₂O₃ + 5 vol. % CO + 5 vol. % O₂ + Fixed bed, 20 (Chen et 44 wt. % 95 vol. % N₂ 95 vol. % N₂ cycles, 950° C. al., 2022) TiO₂ Red mud 43 wt. % Fe₂O₃ 20 vol. % CO + 50 vol. % Fluidized bed, This 80 vol. % N₂ Air + 50 148 cycles, work vol. % N₂ 950° C.

It seems that the other 33 wt. % of inert components (Na, Si, Ca, and Ti) did not impede the outward migration of iron during the chemical looping process. The uniformly distributed Na, Si, Ca, Ti, and Al in pristine RM OC experienced phase segregation after 148 redox cycles. That being said, Na, Si, Ca, Ti, and Al in pristine RM OC were combined in the phases of NaAlSiO₄, CaTiO₃, Fe₃Al₂(SiO₄)₃, and possible FeTiO₃. After 148 redox cycles, the phase of CaTiO₃ segregated noticeably, compared to the CaTiO₃ of pristine RM OC, where the elements of Ca and Ti were coincident in the EDS line-scan pattern. The phase segregation also applies to NaAlSiO₄, as Na and Si were synchronized in the line-scan EDS. Moreover, the phases of NaSiAlO₄, CaTiO₃, and Fe₃Al₂(SiO₄)₃ were identified by XRD patterns as shown in Table 3.

TABLE 3 The detected phases in materials by XRD (XRD patterns are enclosed in supporting information. The JCPDS number for each phase: 1. NaAlSiO4 (#35-0424); 2. CaTiO3 (#22-0153); 3. Fe3Al2(SiO4)3 (#09-0427); 4. FeTiO3 (#29-0733); 5. Fe2O3 (#33-0664); 6. Fe3O4 (#190629); 7. FeO (#06-0615). Samples Detected phases Sample A-Pristine OC Fe₂O₃; NaAlSiO₄; CaTiO₃; Fe₃Al₂(SiO₄)₃; FeTiO₃ * Sample B-Reacted OC Fe₂O₃; NaAlSiO₄; CaTiO₃; Fe₃Al₂(SiO₄)₃ Sample C-Redox fine Fe₂O₃; Fe₃O₄; FeO Sample D-Non-redox fine Fe₂O₃ * This phase is possibly presented.

However, the phase of FeTiO₃ in pristine RM OC is questionable as the main peaks in the XRD pattern of FeTiO₃ (JCPDS #29-0733) are very similar to Fe₂O₃ (JCPDS #33-0664) with about Δ20=1° of peak shift. Besides, the peaks at 20=34.90° and 31.10° of Fe₃Al₂(SiO₄)₃ (JCPDS #09-0427) in pristine RM OC decreased in cycled RM OC, which suggests that the Al in the phase of Fe₃Al₂(SiO₄)₃ probably transformed into NaSiAlO₄ after redox cycles, but the main phase of Fe in RM OC was still Fe₂O₃. Therefore, after redox cyclic tests, the phases of NaSiAlO₄ and CaTiO₃ segregated, which may create grain boundary defects or interstitial defects for the migration of iron ions in the redox cycles of CLC.

The mechanism for the outward migration of iron is as follows. Firstly, the homogenous pristine OC has fully oxidized iron oxides (Fe₂O₃), then Fe₂O₃ on the superficial layer reacts with CO to form reduced iron/oxides. Because no steam was involved in the reaction, and the partial pressure of CO was higher than CO₂, the most likely reactions are Reaction 1+Reaction 2 or Reaction 1+Reaction 2+Reaction 3.

3Fe₂O₃+CO ↔2Fe₃O₄+CO₂  Reaction 1

Fe₃O₄+CO↔3FeO+CO₂  Reaction 2

FeO+CO↔Fe+CO₂  Reaction 3

The detected phases by XRD pattern in Table 3 indicates that the reduction stopped at FeO with 20% CO for only 20 minutes of reduction, which agrees with previous research. In addition, the reduction of Fe₂O₃ experienced volume shrinkage as indicated in Table 4, the molar volume (Fe-based) decreased by 18% from 15.25 cm³/mol Fe in Fe₂O₃ to 12.5 cm³/mol Fe in FeO.

TABLE 4 The molar volume of iron oxides. (Fan, 2017) FeO_(x)-Molar volume Fe-Molar volume Metal oxide cm³/mol_FeO_(x) cm³/mol_Fe Fe₂O₃ 30.5 15.25 Fe₃O₄ 44.7 14.9 FeO 12.5 12.5 Fe 7.1 7.1

This Fe-based volume shrinkage would generate cracks and pores in the core and on the superficial layer of OC particles during the reduction process, which could explain why attrition was dominated by chemical stress during redox reaction (FIG. 4 ). During the oxidation cycle the reduced FeO will be oxidized to Fe₂O₃ with volume expansion. Within a small interparticle gap, the decreased volume of the microparticles in reduction will expand or swell at the normal direction of microparticles during oxidation, resulting in the filling, agglomeration, or bridging of the superficial layer of particles.

Besides, the exothermic oxidation can increase the local temperature, leading to blockage via thermal expansion. The blocked cracks and pores would hinder the inward migration of oxygen. So the reduced iron oxides from the last reduction in the sublayer and core will migrate outward because of the iron concentration gradient and tendency to react with oxygen. After 148 redox cycles in the fluidized bed reactor, the thickness of the iron-enriched layer increased to 5 μm, while leaving the porous interior bulk because of the outward migration of iron. Lastly, the sublayer FeO_(x) expansion/extrusion pushed out Fe₂O₃, where in addition to the mechanical induced stress of the particles, the superficial layer of iron oxides peeled off where surface cracks and attrition exist, and the abrased fines are enriched iron oxides.

Each of the following terms written in singular grammatical form: “a”, “an”, and “the”, as used herein, means “at least one”, or “one or more”. Use of the phrase “One or more” herein does not alter this intended meaning of “a”, “an”, or “the”. Accordingly, the terms “a”, “an”, and “the”, as used herein, may also refer to, and encompass, a plurality of the stated entity or object, unless otherwise specifically defined or stated herein, or, unless the context clearly dictates otherwise. For example, the phrase: “an oxidizing agent”, as used herein, may also refer to, and encompass, a plurality of oxidizing agents.

Each of the following terms: “includes”, “including”, “has”, “having”, “comprises”, and “comprising”, and, their linguistic/grammatical variants, derivatives, or/and conjugates, as used herein, means “including, but not limited to”, and is to be taken as specifying the stated component(s), feature(s), characteristic(s), parameter(s), integer(s), or step(s), and does not preclude addition of one or more additional component(s), feature(s), characteristic(s), parameter(s), integer(s), step(s), or groups thereof.

The phrase “consisting of”, as used herein, is closed-ended and excludes any element, step, or ingredient not specifically mentioned. The phrase “consisting essentially of”, as used herein, is a semi-closed term indicating that an item is limited to the components specified and those that do not materially affect the basic and novel characteristic(s) of what is specified.

Terms of approximation, such as the terms about, substantially, approximately, etc., as used herein, refers to ±10% of the stated numerical value.

The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled. 

What is claimed:
 1. A method of recovering enriched iron fines from bauxite waste, comprising: calcining particles of bauxite waste to form oxygen carrier particles; subjecting the oxygen carrier particles to chemical looping combustion at a temperature of about 950° C.-1,050° C. to produce (a) energy and (b) the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition; and collecting the enriched iron fines.
 2. The method of claim 1, wherein the calcining of the particles of bauxite waste includes heating the particles of bauxite waste to a predetermined temperature for a predetermined period of time needed to form the oxygen carrier particles.
 3. The method of claim 2, wherein the subjecting of the oxygen carrier particles to chemical looping combustion includes cyclically reducing and oxidizing the oxygen carrier particles to produce the enriched iron fines while generating energy.
 4. The method of claim 3, wherein (a) the reducing includes subjecting the oxygen carrier particles in a reactor bed to the presence of a reducing agent, and (b) the oxidizing includes subjecting the oxygen carrier particles in the reactor bed to heating in the presence of an oxidizing agent.
 5. The method of claim 4, wherein the reducing agent is carbon monoxide (CO), H₂, carbon, coal, natural gas or other fuels including solid and gas fuels.
 6. The method of claim 4, further including purging the reactor bed with an inert gas between reducing and oxidizing.
 7. The method of claim 4, wherein the oxidizing agent is air or oxygen mixed with a type of inert gas at greater than 4 vol %.
 8. The method of claim 7, wherein the collecting of the iron enriched fines is done by using a gas-solid separation device, a baghouse, an electrostatic precipitator, a cyclone or a combination thereof.
 9. The method of claim 4, wherein the heating or cooling via thermal cycle of the oxygen carrier particles during the reducing and the oxidizing cycles is to a temperature of between about 950° C. and about 1050° C.
 10. The method of claim 1, wherein (a) the reducing includes subjecting the oxygen carrier particles in a reactor bed to heating, cooling or maintaining temperature in the presence of a reducing agent, and (b) the oxidizing includes subjecting the oxygen carrier particles in the reactor bed to heating in the presence of an oxidizing agent.
 11. The method of claim 10, wherein the heating of the oxygen carrier particles during the reducing and the oxidizing cycles is to a temperature of between about 950° C. and about 1050° C.
 12. The method of claim 1, wherein the subjecting of the oxygen carrier particles to chemical looping combustion includes cyclically reducing and oxidizing the oxygen carrier particles to produce the enriched iron fines while generating energy.
 13. The method of claim 12, wherein the subjecting of the oxygen carrier particles to chemical looping combustion includes cyclically reducing and oxidizing the oxygen carrier particles to produce the enriched iron fines while generating energy.
 14. The method of claim 13, wherein (a) the reducing includes subjecting the oxygen carrier particles in a reactor bed to the presence of a reducing agent, and (b) the oxidizing includes subjecting the oxygen carrier particles in the reactor bed to heating in the presence of an oxidizing agent.
 15. The method of claim 14, wherein the reducing agent is carbon monoxide (CO), H₂, carbon, coal, natural gas or other fuels including solid and gas fuels.
 16. The method of claim 14, wherein the oxidizing agent is air
 17. The method of claim 16, wherein the collecting of the iron enriched fines is done by gas-solid separation devices such as baghouse, ESP, or cyclone.
 18. The method of claim 14, wherein the heating or cooling via thermal cycle of the oxygen carrier particles during the reducing and the oxidizing cycles is to a temperature of between about 950° C. and about 1050° C.
 19. A method of recovering enriched iron fines from bauxite waste, comprising: calcining particles of bauxite waste to form oxygen carrier particles; subjecting the oxygen carrier particles to chemical looping combustion at a temperature above 1000° C. to produce (a) a spent flue gas and (b) the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition; collecting the enriched iron fines from the spent flue gas; and using the spent flue gas to generate steam for power generation.
 20. A method of recovering enriched iron fines from bauxite waste, comprising: subjecting the oxygen carrier particles made from bauxite waste to chemical looping combustion at a temperature above 1000° C. to produce (a) a spent flue gas and (b) the enriched iron fines as a by-product from the oxygen carrier particles via natural attrition; collecting the enriched iron fines; and using the spent flue gas, following the collecting of the enriched iron fines, to generate steam for power generation. 